A competitive-inhibition ELISA (CI-ELISA), based on a monoclonal antibody to an epitope conserved among malignant catarrhal fever virus (MCFV) strains of both wildebeest and sheep origin, was used to determine the prevalence of antibody to MCFV in selected domestic and wild ruminants, both free-ranging and captive, from the USA. We evaluated 2528 sera from 14 species between 1990 and 1995, including 80 pronghorn antelope (Antilocapra americana), 339 bighorn sheep (Ovis canadensis), 103 bison (Bison bison), 17 black-tailed deer (Odocoileus hemionus columbianus), 395 domestic cattle (Bos taunts), 291 domestic goats (Capra hircus), 680 domestic sheep (Ovis ammon), 323 elk (Cervus elaphus), 41 llamas (Lama glama), 21 mouflon sheep (Ovis musimon), 54 mountain goats (Oreamnos americanus), 101 mule deer (Odocoileus hemionus), 20 muskox (Ovibos moschatus), and 63 white-tailed deer (Odocoileus virginianus). A high seroprevalence (37 to 62%) was observed in domestic sheep, domestic goats, muskox, and some bighorn sheep populations. Seroprevalence in these species was generally age-related: a very low seroprevalence was present in these animals under one year of age. A low seroprevalence (2% to 13%) was found in clinically-susceptible species such as domestic cattle, deer, elk and bison, supporting the concept that significant numbers of non-lethal infections occur among clinically susceptible ruminants.